Magit used to use Magit-Popup to implement the transient popup menus. Now it used Transient instead, which is Magit-Popup’s successor.
In the older Magit-Popup menus, it was possible to save user settings
(e.g., setting the gpg signing key for commits) by using C-c C-c in
the popup buffer. This would dismiss the popup, but save the settings
as the defaults for future popups.
When switching to Transient menus, this functionality is now available
via C-x C-s instead; the C-x prefix has other options as well when
using Transient, which will be displayed when it is typed. See
https://magit.vc/manual/transient/Saving-Values.html#Saving-Values for
more details.